1. Prentice Hall © 20041 Chapter 6: Joins SQL for SQL Server Bijoy Bordoloi and Douglas Bock

2. Prentice Hall © 20042 OBJECTIVES Learn the basic join operation rules.Learn the basic join operation rules. Write equijoin and inequality join queries by using the WHERE clause.Write equijoin and inequality join queries by using the WHERE clause. Write equijoin and inequality join queries by using the JOIN keyword in the FROM clause.Write equijoin and inequality join queries by using the JOIN keyword in the FROM clause. Write complex join queries with more than two tables and more than two columns.Write complex join queries with more than two tables and more than two columns. Write outer join queries.Write outer join queries. Write self join (recursive) queries.Write self join (recursive) queries.

3. Prentice Hall © 20043 A TYPICAL JOIN OPERATION The following figure displays the employee and department tables.The following figure displays the employee and department tables. The figure illustrates the concept of a JOIN operation by connecting columns within each table with a line.The figure illustrates the concept of a JOIN operation by connecting columns within each table with a line. One line connects the employee table’s emp_dpt_number column with the department table's dpt_no column.One line connects the employee table’s emp_dpt_number column with the department table's dpt_no column. A second line connects the employee table’s emp_ssn column to the department table’s dep_mgrssn column.A second line connects the employee table’s emp_ssn column to the department table’s dep_mgrssn column.

4. Prentice Hall © 20044 A TYPICAL JOIN OPERATION

5. Prentice Hall © 20045 JOINS We will begin our study of JOIN operations by focusing on the relationship between the employee and department tables represented by the common department number values.We will begin our study of JOIN operations by focusing on the relationship between the employee and department tables represented by the common department number values. Our first query lists employee names and department numbers. This query only retrieves data from the single employee table.Our first query lists employee names and department numbers. This query only retrieves data from the single employee table.

6. Prentice Hall © 20046 JOINS Contd. SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_dpt_number As CHAR(4)) "Dept" CAST(emp_dpt_number As CHAR(4)) "Dept" FROM employee; Last Name First Name Dept ------------ ------------ ---- Bock Douglas 7 Amin Hyder 3 Joshi Dinesh 7 Zhu Waiman 7 more rows will be displayed...

7. Prentice Hall © 20047 JOINS Contd. A large organization can have dozens or even hundreds of departments. Thus, the numbers displayed in the department column shown above may not be very meaningful.A large organization can have dozens or even hundreds of departments. Thus, the numbers displayed in the department column shown above may not be very meaningful. Suppose you want the department names instead of the department numbers to be listed.Suppose you want the department names instead of the department numbers to be listed. The department names are mentioned in the “Department” table.The department names are mentioned in the “Department” table. Hence we need to join the “Employee” and the “Department” tables to get the required results.Hence we need to join the “Employee” and the “Department” tables to get the required results.

8. Prentice Hall © 20048 JOINS Contd. Joins can be specified in either the FROM or WHERE clauses.Joins can be specified in either the FROM or WHERE clauses. The join conditions combine with the search conditions in the WHERE and HAVING clauses to control the rows that are selected from the tables referenced in the FROM clause.The join conditions combine with the search conditions in the WHERE and HAVING clauses to control the rows that are selected from the tables referenced in the FROM clause. Early standards for SQL specified the use of a WHERE clause to join tables. The newer ANSI/ISO SQL-99 standard uses the FROM clause to carry out a JOIN operation.Early standards for SQL specified the use of a WHERE clause to join tables. The newer ANSI/ISO SQL-99 standard uses the FROM clause to carry out a JOIN operation. The older syntax is, however, still valid, and is expected to remain valid in the future (except for OUTER JOIN).The older syntax is, however, still valid, and is expected to remain valid in the future (except for OUTER JOIN).

9. Prentice Hall © 20049 JOINS Contd. /* JOIN using the WHERE clause */ SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee, department WHERE emp_dpt_number = dpt_no; /* JOIN using the FROM clause */ SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no); Output is identical for both the queries.Output is identical for both the queries.

10. Prentice Hall © 200410 JOINS Contd. Last Name First Name Department Name ------------ ------------ --------------- Bock Douglas Production Amin Hyder Admin and Records Joshi Dinesh Production Zhu Waiman Production more rows will be displayed...

11. Prentice Hall © 200411 HOW JOINS ARE PROCESSED Conceptually, when two tables are joined, SQL creates a Cartesian product of the tables.Conceptually, when two tables are joined, SQL creates a Cartesian product of the tables. A Cartesian product consists of all possible combinations of the rows from each of the tables regardless of whether or not the rows are related to one another.A Cartesian product consists of all possible combinations of the rows from each of the tables regardless of whether or not the rows are related to one another. Therefore, when a table with 10 rows is joined with a table with 20 rows, the Cartesian product is 200 rows (10 x 20 = 200 ).Therefore, when a table with 10 rows is joined with a table with 20 rows, the Cartesian product is 200 rows (10 x 20 = 200 ).

12. Prentice Hall © 200412 HOW JOINS ARE PROCESSED Contd. Following are two tables simply named Table_1 and Table_2.Following are two tables simply named Table_1 and Table_2. Each table has a single column named Col_1. Each table also has three rows with simple alphabetic values stored in the Col_1 column.Each table has a single column named Col_1. Each table also has three rows with simple alphabetic values stored in the Col_1 column. Table_1 Table_2 Table_1 Table_2 COL_1 COL_1 a b c a b c

13. Prentice Hall © 200413JOINS SELECT * FROM table_1, table_2; COL_1 COL_1 -------- --------- a a b a c a a b b b c b a c b c cc

14. Prentice Hall © 200414 JOINS Contd. The first row of the “table_1” table was joined with every row in the “table_2” table.The first row of the “table_1” table was joined with every row in the “table_2” table. A Cartesian product may not be useful and could be misleading.A Cartesian product may not be useful and could be misleading. Always include a WHERE clause in your JOIN statements.Always include a WHERE clause in your JOIN statements. SELECT * FROM table_1, table_2; WHERE table_1.col_1 = table_2.col_1; col_1 col_1 col_1 col_1 -------- -------- -------- -------- a a a a b b b b c c c c

15. Prentice Hall © 200415 JOIN Example Using the WHERE clauseUsing the WHERE clause SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e, department d WHERE e.emp_dpt_number = d.dpt_no AND e.emp_dpt_number = 7; AND e.emp_dpt_number = 7; Last Name First Name Department Name ------------ ------------ -------------------- Bock Douglas Production Joshi Dinesh Production Zhu Waiman Production Prescott Sherri Production

16. Prentice Hall © 200416 JOIN Example Contd. Using the FROM clauseUsing the FROM clause SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no) AND e.emp_dpt_number = 7; AND e.emp_dpt_number = 7;

17. Prentice Hall © 200417 JOIN Example Contd. Using a combination of FROM and WHERE clauses (recommended method)Using a combination of FROM and WHERE clauses (recommended method) SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no) WHERE e.emp_dpt_number = 7;

18. Prentice Hall © 200418 JOIN OPERATION RULES JOINS and the SELECT Clause A JOIN query always begins with a SELECT clause.A JOIN query always begins with a SELECT clause. List the columns to be displayed in the result table after the SELECT keyword.List the columns to be displayed in the result table after the SELECT keyword. The result table column order reflects the order in which column names are listed in the SELECT clause.The result table column order reflects the order in which column names are listed in the SELECT clause. To modify the order in which column names are listed, simply rearrange the order of the column listing in the SELECT clause.To modify the order in which column names are listed, simply rearrange the order of the column listing in the SELECT clause.

19. Prentice Hall © 200419Example SELECT dpt_name "Department Name", CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name" CAST(emp_first_name As CHAR(12)) "First Name" FROM employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no) WHERE e.emp_dpt_number = 7; Department Name Last Name First Name -------------------- ------------ ------------ Production Bock Douglas Production Joshi Dinesh Production Zhu Waiman Production Prescott Sherri

20. Prentice Hall © 200420 JOIN OPERATION RULES JOIN operations also support the specification of all columns by the use of a simple asterisk (*) in a SELECT clause.JOIN operations also support the specification of all columns by the use of a simple asterisk (*) in a SELECT clause. The result table for the query will contain all columns from both the tables.The result table for the query will contain all columns from both the tables. When the asterisk (*) is used, the column order of the result table is based on the order in which tables are listed in the FROM clause.When the asterisk (*) is used, the column order of the result table is based on the order in which tables are listed in the FROM clause.

21. Prentice Hall © 200421 JOINS and the FROM Clause Any SELECT statement that has two or more table names listed in a FROM clause is a JOIN query.Any SELECT statement that has two or more table names listed in a FROM clause is a JOIN query. By definition, a JOIN operation retrieves rows from two or more tables.By definition, a JOIN operation retrieves rows from two or more tables. The FROM clause is always used to list the tables from which columns are to be retrieved by a JOIN query.The FROM clause is always used to list the tables from which columns are to be retrieved by a JOIN query. The FROM clause listing has a limit of 256 table names.The FROM clause listing has a limit of 256 table names. The order of table name listings is irrelevant to the production of the result table with the one exception – that is, if you use an asterisk (*) in the SELECT clause, then the column order in the result table reflects the order in which tables are listed in the FROM clause.The order of table name listings is irrelevant to the production of the result table with the one exception – that is, if you use an asterisk (*) in the SELECT clause, then the column order in the result table reflects the order in which tables are listed in the FROM clause.

22. Prentice Hall © 200422 JOINS and the FROM Clause Contd. As explained earlier, you can specify the join conditions in a FROM clause by explicitly using the JOIN clause within the FROM clause.As explained earlier, you can specify the join conditions in a FROM clause by explicitly using the JOIN clause within the FROM clause. You can also specify the type of JOIN such as INNER, LEFT OUTER, RIGHT OUTER or FULL OUTER.You can also specify the type of JOIN such as INNER, LEFT OUTER, RIGHT OUTER or FULL OUTER. If you do not specify the type of join then, by default, the join operation is always an INNER join.If you do not specify the type of join then, by default, the join operation is always an INNER join. We did not indicate the type of join in any of the example queries thus far. So, they are all examples of INNER join operations.We did not indicate the type of join in any of the example queries thus far. So, they are all examples of INNER join operations. The various OUTER join operations are covered later in this chapter.The various OUTER join operations are covered later in this chapter.

23. Prentice Hall © 200423 JOINS and the WHERE Clause The WHERE clause specifies the relationship between tables listed in the FROM clause.The WHERE clause specifies the relationship between tables listed in the FROM clause. It also restricts the rows displayed in the result table.It also restricts the rows displayed in the result table. T he order of selection criteria or JOIN operations is not important.T he order of selection criteria or JOIN operations is not important.

24. Prentice Hall © 200424 JOINS and the WHERE Clause Contd. SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e, department d WHERE e.emp_dpt_number = 7 AND e.emp_dpt_number = d.dpt_no; Last Name First Name Department Name ------------ ------------ -------------------- Bock Douglas Production Joshi Dinesh Production Zhu Waiman Production Prescott Sherri Production

25. Prentice Hall © 200425 JOINS and the WHERE Clause Contd. A note of caution though! The scenario exemplified in the previous example applies only if you are joining tables using the WHERE clause. If you elect to both join tables and specify row selection criteria in a FROM clause, then you MUST specify the JOIN operation before specifying the row selection criteria. Otherwise, SQL Server will generate an error message. SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM emp_dpt_number = 7 and employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no); employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no); Server: Msg 170, Level 15, State 1, Line 5 Line 5: Incorrect syntax near '='.

26. Prentice Hall © 200426 QUALIFYING COLUMN NAMES Normally you do not need to create alias names for tables. When column names are ambiguous (the column names used are from more than one table) you must qualify themNormally you do not need to create alias names for tables. When column names are ambiguous (the column names used are from more than one table) you must qualify them SELECT * FROM table_1, table_2 WHERE col_1 = col_1; This query is WRONG, SQL Server would reject this query and generate an error message.This query is WRONG, SQL Server would reject this query and generate an error message. Server: Msg 209, Level 16, State 1, Line 2 Ambiguous column name 'Col_1'.

27. Prentice Hall © 200427 QUALIFYING COLUMN NAMES Contd. This error message tells you that you have included a column name somewhere in the query that exists in more than one table listed in the FROM clause.This error message tells you that you have included a column name somewhere in the query that exists in more than one table listed in the FROM clause. Here the error is in the WHERE clause; however, it is also possible to make a similar error in the SELECT clause.Here the error is in the WHERE clause; however, it is also possible to make a similar error in the SELECT clause. The SELECT statement shown below fails to qualify the col_1 name in the SELECT clause, and SQL Server again produces an error message.The SELECT statement shown below fails to qualify the col_1 name in the SELECT clause, and SQL Server again produces an error message. SELECT col_1 FROM table_1, table_2 WHERE table_1.col_1 = table_2.col_1; Server: Msg 209, Level 16, State 1, Line 2 Ambiguous column name 'Col_1'.

28. Prentice Hall © 200428 QUALIFYING COLUMN NAMES Contd. An ambiguous column name is qualified by using the DOT (.) connector to connect the table name and column name.An ambiguous column name is qualified by using the DOT (.) connector to connect the table name and column name. Sometimes it is easier to qualify column names by using table alias names.Sometimes it is easier to qualify column names by using table alias names. Often, a single letter is used as an identifier to reduce keystroke requirements.Often, a single letter is used as an identifier to reduce keystroke requirements. SELECT CAST(e.emp_last_name As CHAR(12)) "Last Name", CAST(e.emp_first_name As CHAR(12)) "First Name", CAST(e.emp_first_name As CHAR(12)) "First Name", dpt_name "Department Name" dpt_name "Department Name" FROM employee e JOIN department d ON (e.emp_dpt_number = d.dpt_no) WHERE e.emp_dpt_number = 7;

29. Prentice Hall © 200429 QUALIFYING COLUMN NAMES Contd. The use of the letters “e” and “d” is completely arbitrary; “t1” and “t2” or any other unique aliases could have been used.The use of the letters “e” and “d” is completely arbitrary; “t1” and “t2” or any other unique aliases could have been used. The important points to learn are:The important points to learn are: –The alias must follow a table name. –Use a space to separate a table name and its alias. –The alias must be unique within the SELECT statement. If the column names are not identical you are not required to qualify them, although you still might want to for documentation purposes.If the column names are not identical you are not required to qualify them, although you still might want to for documentation purposes.

30. Prentice Hall © 200430 JOIN Operations Using Inequality Operators ( ) The most commonly used JOIN operator is the "equal" (=) sign.The most commonly used JOIN operator is the "equal" (=) sign. You may use any relational operator in a JOIN query. The next query uses an inequality operator, the greater than (>) relational operator.You may use any relational operator in a JOIN query. The next query uses an inequality operator, the greater than (>) relational operator. SELECT CAST(emp_last_name As CHAR(12)) "Emp Last Name", CAST(dep_name AS CHAR(12)) "Dependent Name" CAST(dep_name AS CHAR(12)) "Dependent Name" FROM employee e JOIN dependent d ON (e.emp_ssn <> d.dep_emp_ssn) WHERE e.emp_last_name IN ('Bordoloi', 'Bock') ORDER BY emp_last_name; Emp Last Name Dependent Name ------------- -------------- Bock Jo Ellen.... Bock Rita Bordoloi Deanna More rows are displayed..

31. Prentice Hall © 200431 Joining More Than Two Tables While the examples given thus far have joined rows from two tables, you can specify up to 256 tables in a JOIN operation for SQL Server.While the examples given thus far have joined rows from two tables, you can specify up to 256 tables in a JOIN operation for SQL Server. The more tables that are included in a JOIN operation, the longer the query will take to process, especially when the tables are large with millions of rows per table.The more tables that are included in a JOIN operation, the longer the query will take to process, especially when the tables are large with millions of rows per table.

32. Prentice Hall © 200432 Joining More Than Two Tables Contd. The example shown in following figure joins three tables to produce a result table based on two different relationships.The example shown in following figure joins three tables to produce a result table based on two different relationships.

33. Prentice Hall © 200433 Joining More Than Two Tables Contd. ExampleExample SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", '$' + CONVERT (CHAR (12), 1.10*emp_salary, 1) "Raised Salary", '$' + CONVERT (CHAR (12), 1.10*emp_salary, 1) "Raised Salary", p.pro_name "Project" p.pro_name "Project" FROM employee e JOIN assignment a ON (e.emp_ssn = a.work_emp_ssn) JOIN project p ON (a.work_pro_number = p.pro_number) JOIN project p ON (a.work_pro_number = p.pro_number) WHERE p.pro_name = 'Inventory'; Last Name First Name Raised Salary Project ------------ ------------ ------------- --------- Amin Hyder $27500.000000 Inventory Zhu Waiman $47300.000000 Inventory Markis Marcia $27500.000000 Inventory

34. Prentice Hall © 200434 Joining More Than Two Tables Contd. Note that in the previous example you are displaying data only from two tables, employee and project. Yet, you are joining three tables, employee, assignment, and project!Note that in the previous example you are displaying data only from two tables, employee and project. Yet, you are joining three tables, employee, assignment, and project! This is because the tables, employee and project, are not directly joinable as they do not have any direct relationship represented by ‘primary- foreign key’ relationships.This is because the tables, employee and project, are not directly joinable as they do not have any direct relationship represented by ‘primary- foreign key’ relationships.

35. Prentice Hall © 200435 Joining More Than Two Tables Contd. A typical join condition specifies a foreign key from one table and its associated primary key in the other table. In order to display data from tables linked through an association table, you may need to join three or four or even more tables.A typical join condition specifies a foreign key from one table and its associated primary key in the other table. In order to display data from tables linked through an association table, you may need to join three or four or even more tables. However, you can join only two tables at a time. That is, you can have only two table names in each join condition. Thus, if you are joining two tables then you must have at least one join condition; if you are joining n tables then you must have at least n-1 join conditions!However, you can join only two tables at a time. That is, you can have only two table names in each join condition. Thus, if you are joining two tables then you must have at least one join condition; if you are joining n tables then you must have at least n-1 join conditions!

36. Prentice Hall © 200436 Joining Tables by Using Two Columns The diagram depicts the relationship at a university where students enroll in course sections.The diagram depicts the relationship at a university where students enroll in course sections.

37. Prentice Hall © 200437 Joining Tables by Using Two Columns Contd. The SELECT statement that accomplishes the JOIN based on two columns is shown below.The SELECT statement that accomplishes the JOIN based on two columns is shown below. This situation arises when the related tables have composite primary key columns.This situation arises when the related tables have composite primary key columns. SELECT s.course_title "Course Title", e.student_ssn "Student SSN" FROM enrollment e, section s WHERE e.course_number = s.course_number AND e.section_number = s.section_number; e.section_number = s.section_number; /* Join conditions in the FROM clause */ SELECT s.course_title "Course Title", e.student_ssn "Student SSN" FROM enrollment e JOIN section s ON (e.course_number = s.course_number) AND ON (e.course_number = s.course_number) AND (e.section_number = s.section_number); (e.section_number = s.section_number);

38. Prentice Hall © 200438 OUTER JOIN Operations SQL Server also supports what is called an outer-join. This means that a row will appear in the joined table even though there is no matching value in the table to be joined.SQL Server also supports what is called an outer-join. This means that a row will appear in the joined table even though there is no matching value in the table to be joined. There are three types of outer joins: LEFT, RIGHT, and FULL. They all begin with an INNER JOIN, and then they add back some of the rows that have been dropped.There are three types of outer joins: LEFT, RIGHT, and FULL. They all begin with an INNER JOIN, and then they add back some of the rows that have been dropped.

39. Prentice Hall © 200439 OUTER JOIN OPERATIONS:Types of OUTER JOIN Contd. A LEFT OUTER JOIN adds back all the rows that are dropped from the first (left) table in the join condition, and output columns from the second (right) table are set to NULL.A LEFT OUTER JOIN adds back all the rows that are dropped from the first (left) table in the join condition, and output columns from the second (right) table are set to NULL. A RIGHT OUTER JOIN adds back all the rows that are dropped from the second (right) table in the join condition, and output columns from the first (left) table are set to NULL.A RIGHT OUTER JOIN adds back all the rows that are dropped from the second (right) table in the join condition, and output columns from the first (left) table are set to NULL. The FULL OUTER JOIN adds back all the rows that are dropped from both the tables.The FULL OUTER JOIN adds back all the rows that are dropped from both the tables.

40. Prentice Hall © 200440 OUTER JOIN Operations CONTD. Suppose you want to know the names of the employees regardless of whether they have dependents or not. You can use the OUTER JOIN as follows:Suppose you want to know the names of the employees regardless of whether they have dependents or not. You can use the OUTER JOIN as follows: SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_relationship As CHAR(8)) "Relationship" CAST(dep_relationship As CHAR(8)) "Relationship" FROM employee LEFT OUTER JOIN dependent ON (emp_ssn = dep_emp_ssn);

41. Prentice Hall © 200441 OUTER JOIN Operations Contd. Last Name First Name Dependent Relationship ------------ ------------ ------------ ------------ Bock Douglas Deanna DAUGHTER Bock Douglas Jeffery SON Bock Douglas Mary Ellen SPOUSE Bock Douglas Michelle DAUGHTER Bock Douglas Rachael DAUGHTER Amin Hyder NULL NULL Joshi Dinesh NULL NULL Zhu Waiman Andrew SON Zhu Waiman Jo Ellen DAUGHTER Zhu Waiman Susan SPOUSE Joyner Suzanne Allen SPOUSE Bordoloi Bijoy Anita DAUGHTER Bordoloi Bijoy Mita SPOUSE Bordoloi Bijoy Monica DAUGHTER Bordoloi Bijoy Rita DAUGHTER Markis Marcia NULL NULL Prescott Sherri NULL NULL (17 row(s) affected)

42. Prentice Hall © 200442 OUTER JOINS and NULL values Management might desire a listing of employees with no dependents to satisfy some governmental reporting requirement.Management might desire a listing of employees with no dependents to satisfy some governmental reporting requirement. We can take advantage of the fact that the dep_name column will be NULL for employees with no dependents, and simply add a criteria to the WHERE clause to include employees where the dep_name column is NULL.We can take advantage of the fact that the dep_name column will be NULL for employees with no dependents, and simply add a criteria to the WHERE clause to include employees where the dep_name column is NULL.

43. Prentice Hall © 200443 OUTER JOINS and NULL values Contd. SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_relationship As CHAR(8)) "Relationship" CAST(dep_relationship As CHAR(8)) "Relationship" FROM employee LEFT OUTER JOIN dependent ON (emp_ssn = dep_emp_ssn) WHERE dep_name IS NULL; Last Name First Name Dependent Relationship ------------ ------------ ------------ ------------ Amin Hyder NULL NULL Joshi Dinesh NULL NULL Markis Marcia NULL NULL Prescott Sherri NULL NULL

44. Prentice Hall © 200444 RIGHT OUTER JOIN SELECT CAST(emp_last_name As CHAR(12)) "Last Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(emp_first_name As CHAR(12)) "First Name", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_name As CHAR(12)) "Dependent", CAST(dep_relationship As CHAR(8)) "Relationship" CAST(dep_relationship As CHAR(8)) "Relationship" FROM dependent RIGHT OUTER JOIN employee ON (emp_ssn = dep_emp_ssn); Last Name First Name Dependent Relationship ------------ ------------ ------------ ------------ Bock Douglas Deanna DAUGHTER Bock Douglas Jeffery SON Bock Douglas Mary Ellen SPOUSE Bock Douglas Michelle DAUGHTER Bock Douglas Rachael DAUGHTER Amin Hyder NULL NULL Joshi Dinesh NULL NULL Zhu Waiman Andrew SON Zhu Waiman Jo Ellen DAUGHTER Zhu Waiman Susan SPOUSE Joyner Suzanne Allen SPOUSE Bordoloi Bijoy Anita DAUGHTER Bordoloi Bijoy Mita SPOUSE Bordoloi Bijoy Monica DAUGHTER Bordoloi Bijoy Rita DAUGHTER Markis Marcia NULL NULL Prescott Sherri NULL NULL (17 row(s) affected)

45. Prentice Hall © 200445 SELF-JOIN Operations A SELF JOIN operation is used to produce a result table when the relationship of interest exists among rows that are stored within a single table.A SELF JOIN operation is used to produce a result table when the relationship of interest exists among rows that are stored within a single table.

46. Prentice Hall © 200446 SELF-JOIN Operations Contd. SELECT CAST(e1.emp_last_name + ', ' + e1.emp_first_name As CHAR(20)) "Supervisor", CAST(e2.emp_last_name + ', ' + e2.emp_first_name As CHAR(20)) CAST(e2.emp_last_name + ', ' + e2.emp_first_name As CHAR(20)) "Employee" "Employee" FROM employee e1 JOIN employee e2 ON (e1.emp_ssn = e2.emp_superssn) ORDER BY e2.emp_superssn; Supervisor Employee -------------------- -------------------- Zhu, Waiman Bock, Douglas Zhu, Waiman Joshi, Dinesh Zhu, Waiman Prescott, Sherri Joyner, Suzanne Markis, Marcia Joyner, Suzanne Amin, Hyder Bordoloi, Bijoy Zhu, Waiman Bordoloi, Bijoy Joyner, Suzanne

47. Prentice Hall © 200447 SUMMARY The JOIN operation enables you to combine data from several tables.The JOIN operation enables you to combine data from several tables. You can specify your join conditions either in the FROM clause (newer syntax) or in the WHERE clause (older syntax).You can specify your join conditions either in the FROM clause (newer syntax) or in the WHERE clause (older syntax). You typically join tables using inner join based on equality of columns (equijoin).You typically join tables using inner join based on equality of columns (equijoin). However, you also learned about some other type of joins such as outer join and self join (recursive).However, you also learned about some other type of joins such as outer join and self join (recursive).